MavEtJu's Distorted View of the World

What does that mean you might wonder, and how is that different
from grey-listing.

With sender verification, postfix checks if the address in the MAIL
FROM command gets accepted by the MX servers of that address. It
tries to do it realtime, but if it takes too long (>6 seconds) it
will temporary fail the SMTP session and waits until the sending
MTA retries.

Grey-listing on the other just temporary fails the first delivery
attempt and waits until the sending MTA retries.

Keep in mind that two different things are checked here:

Existence of the (well, acceptance of email for the) sender email address at the MX host.

Proper handling of forced retries by the sending MTA.

One day I'll be brave enough to do also SPF checking (Allowed
delivery of email for that domain by that MTA) and everything is
as open as it will be, or as closed as it will be.